In general, a spark plug used for an internal combustion engine, such as a gasoline engine, includes a center electrode, an insulator provided around the center electrode, a metallic shell provided around the insulator, and a ground electrode (also called “outer electrode”) attached to the metallic shell so as to form a spark discharge gap in cooperation with the center electrode.
The ground electrode and the center electrode, which are components of the spark plug (hereinafter collectively referred as “electrodes”) are manufactured through extrusion forming in which a starting material (workpiece) of the electrodes is passed through a die. More specifically, the manufacture of electrodes is performed through the steps of supplying a single workpiece to the die, pushing the workpiece by use of a punch so as to pass the workpiece through the die, removing the extruded workpiece (hereinafter called an “extrudate”) from the die, and performing cutting work or the like. For example, see Japanese Patent Application Laid-Open (kokai) No. H4-319283 (“Patent Document 1”) and Japanese Patent Application Laid-Open (kokai) No. H4-294085 (“Patent Document 2”).